In the year 2274, the remnants of human civilization live in a sealed domed city, a utopia run by a computer that takes care of all aspects of their life, including reproduction. The citizens live a hedonistic life but, to maintain the city, everyone must undergo the ritual of "Carrousel" when they reach the age of 30. There, they are vaporized and ostensibly "renewed". To track this, they are implanted at birth with a "life-clock" crystal in the palm of their hand that changes color as they approach their "Last Day." Most residents accept this promise of rebirth, but those who do not are known as "Runners" and "Sandmen" are assigned to pursue and terminate them.
Logan 5 and Francis 7 are partner Sandmen; after terminating a Runner, Logan finds an ankh among his possessions. Later, he meets Jessica 6, a girl also wearing an ankh pendant. Logan takes the ankh to the computer, where he is told it is a symbol for a secret group who helps the Runners find "Sanctuary". The computer instructs Logan to find Sanctuary and destroy it. It then changes the color of his life-clock to flashing red, four years early. In order to escape Carrousel, Logan is now forced to become a Runner. Logan meets Jessica and explains his situation. They meet with the underground group that leads them to the periphery of the city. Logan finds the ankh symbol unlocks an exit from the city. They come out into a frozen cave, with Francis following closely behind. In the cave, they meet Box, a robot designed to capture food for the city from the outside. Box also captures escaped Runners and freezes them like artwork. Before he can freeze them, Logan and Jessica escape from the robot, causing the cave to collapse on Box.
Once outside, Logan and Jessica notice that their life-clocks are dead. They discover that the wilderness has overrun the remains of human civilization. They explore a city, which was once Washington D.C.. In the ruins of the United States Senate chamber, they discover an elderly man. His appearance is a shock to them since neither has seen anyone over the age of 30. The old man explains what happened to humanity outside of the city and the fugitives realize Sanctuary is a myth. Francis has followed them and he and Logan fight. Logan fatally wounds Francis and as he dies, he sees that Logan's life-clock is now clear and assumes that Logan has renewed. Logan and Jessica convince the old man to return to the city with them. Leaving the man outside, the two enter and try to convince everyone that Carrousel is a lie and not necessary. The two are captured by other Sandmen and taken to the computer. The computer interrogates Logan and asks if he completed his mission but Logan insists "there is no Sanctuary". This answer is not accepted by the computer, even after scanning Logan's mind, and the computer overloads, causing the city's systems to fail and release the exterior seals. Logan and Jessica return to the old man as the citizens flee the ruined city. When the citizens are outside they see the old man, who is the first human they see far older than 30, proving that they can live their lives much longer, bringing them freedom.